1. Field of the Invention
The invention relates to ballasts for lamps and, in particular, to electronic ballasts for discharge lamps.
2. Description of Related Art
It has been estimated that over 25% of all electrical energy consumed in the world is utilized to power artificial lighting. Thus, the importance of efficient electrical lighting sources cannot be understated.
The most efficient electrical lighting sources, that are commonly available, are gaseous low-pressure and high-pressure discharge lamps, e.g. fluorescent and high-intensity-discharge (HID) lamps, respectively. These types of lamps typically have a negative-resistance characteristic and are driven by current-limiting circuits known as ballasts.
Two general types of lamp ballasts, i.e. electromagnetic and electronic, are in common usage for driving discharge lamps. Electromagnetic ballasts have only passive circuit components and typically drive lamps at power-line frequencies. Electronic ballasts include both passive and active circuit components and typically drive lamps at frequencies much higher than power line frequencies. Generally, the electromagnetic ballast is less expensive. However, the electronic ballast is smaller and lighter, operates discharge lamps more efficiently, with less audible noise and with no visible flicker, and contributes to a longer lamp life. Additionally, electronic ballasts can regulate discharge-lamp power more effectively than electromagnetic ballasts in response to changing power-line and lamp-operating conditions.
FIG. 1 schematically illustrates a typical electronic ballast for driving a discharge lamp L from an AC voltage v.sub.ac provided by a power source PS, such as power lines from a local utility. The ballast includes an electromagnetic interference filter EMI, a full-wave rectifier bridge BR, a power-factor correction circuit, an energy-storage capacitor C.sub.e, and a half-bridge resonant inverter, all electrically connected in series between the power source and the lamp L.
The filter EMI prevents electromagnetic interference generated by the ballast circuitry and the discharge lamp from being conducted back to the power source. The ballast generates EMI in the form of high harmonic currents which, if conducted back to the power source, could cause problems such as excessive neutral currents, overheated transformers, and interference with any sensitive electronic equipment which also receives electrical energy from the power source. Discharge lamps can generate both electromagnetic and radio-frequency interference.
The power-factor correction circuit is a well-known boost-converter type, including an inductor L.sub.10, a switching transistor Q.sub.10 and a diode D.sub.10. It functions to increase the power factor at the AC power source and to boost the DC voltage provided to the energy-storage capacitor C.sub.e by the bridge BR.
The energy-storage capacitor C.sub.e performs two functions. First, it acts as a DC voltage source for the resonant inverter. Second, it balances the energy flow between the load and the power source. When the AC power source PS is instantaneously supplying less power than the load is consuming, C.sub.e must deliver energy to the load. Conversely, when the AC power source is instantaneously supplying more power than the load is consuming, C.sub.e must store energy.
The resonant inverter output stage converts the DC voltage on the energy storage capacitor C.sub.e to a high frequency sinusoidal drive for the lamp L. Two transistor switches Q.sub.11 and Q.sub.12 are electrically connected in a half-bridge configuration and operate at a 50% duty cycle to chop the DC voltage into a high-frequency square wave. A capacitor C.sub.b is provided for blocking DC components from reaching a transformer T, which is provided for both isolation and impedance matching. An inductor L.sub.f and a capacitor C.sub.f form a second-order filter for filtering the high-frequency square wave so that a sinusoidal voltage and current, at the fundamental switching frequency, are applied to the discharge lamp L.